In many generators, such as high speed generators utilized in aircraft, it is highly desirable to eliminate brushes since they frequently require maintenance and/or replacement, and are perhaps the single weakest point in the system in terms of breakdowns. Moreover, by its very nature, the electrical path between a brush and a commutator is subject to arcing which may introduce transients into the electrical energy being produced, which in turn, may interfere with proper operation of some types of electrical loads on the generator.
A typical brushless generator has three distinct generating systems including a main generator, an exciter, and a permanent magnet generator. The permanent magnet generator includes permanent magnets for establishing a magnetic field which is employed to induce current in a set of windings, which in turn is employed after rectification to generate a magnetic field in the exciter. The magnetic field in the exciter, is in turn, employed to induce an even higher level of current, typically three-phase alternating current, which is then employed to generate the magnetic field for the main generator by flowing the current through the main field winding of the generator system.
In order to avoid the use of brushes, it is necessary that the magnetic field in the main generator be in the rotor so that the output of the system can be taken from the stator of the main generator. In order to generate a suitable magnetic field in the rotor, it is necessary to utilize direct current, as opposed to alternating current, for the same. Since the output of the exciter is an alternating current, this current must be rectified to direct current. And, again, in order to avoid resort to brushes, it is accordingly necessary that the rectifier interconnecting the exciter in the main generator field winding be carried by the rotor of the generator. The U.S. Pat. No. 3,577,002 issued May 4, 1971 to Hall et al, is generally illustrative of this approach although it does show a commutator and brush structure which is necessary for alternative use of the generating system as a starter motor. The same approach is also illustrated in U.S. Pat. No. 4,139,789 issued Feb. 13, 1979 to Hunt. Furthermore, an example of a rotating rectifier assembly specifically intended for use in such systems is illustrated in the commonly assigned application of Trommer, application Ser. No. 635,511 filed July 30, 1984 now U.S. Pat. No. 4,603,344, and entitled "Rotating Rectifier Assembly".
In any event, because the rectifier assembly is carried by the rotating shaft of the generating system, it is subjected to high centrifugal loading and much care must be taken to assure that the components of the rectifier are adequately supported against such forces. It will also be appreciated that it is highly desirable to minimize the space occupied by the rectifier assembly since it is typically housed within the rotor shaft itself. Finally, it is desirable to make provision for cooling the rectifier assembly during operation since such not only minimizes the possibility of thermal damage to the components, but provides a greater capacity over correspondingly sized, uncooled rectifier assembly.
It is also desirable that the assembly be such that it may be easily removed for servicing in the event of component failure.
The present invention is, therefore, directed to the provision of a semiconductor assembly, typically a rectifier assembly, having the foregoing attributes.